The present invention relates to an automatic gain control system for maintaining a constant output level by attenuating or amplifying an input signal, and more particularly to a digital automatic gain control system.
As a prior automatic gain control system, there is provided a system which was disclosed in the 138th page of "Electronic Design" issued on Sept. 16, 1982. The prior automatic gain control system shown in FIG. 1, comprises a step gain controller 1 adapted to provide a variable gain control for controlling an output signal level in response to an input signal level from an input line 8, a buffer 2 adapted not to influence the step gain controller 1 upon any variation of signals through an output line 9, a rectifying detector means 10 for detecting and thereby outputting a constant direct-current (dc) voltage corresponding to a signal level on the output line 9, a window comparator 11 for providing a specified logic level by means of comparing said dc voltage with fixed reference voltages in both nodes 13 and 14 respectively divided from a dc supply voltage received in a reference voltage input terminal 12, a clock pulse generator 7 and a up/down counter 6 for performing an up or down counting operation and stopping the count upon receiving the logic output of said window comparator 11 and the clock pulse. Accordingly, when the signal level of the output line 9 goes over a specified level, the up/down counter 6 begins a down counting on the basis of a logic output of said window comparator 11, and when being on the contrary (i.e., below a specified level), it begins an up counting. However, when the signal level is within the range between two fixed levels, it stops the count. By these steps, the output signal level of the output line 9 can be constantly maintained in a specified range between the two levels.
However, the prior automatic gain control system as described hereinbefore is a semi-analog system because the rectifying detection means 10 operates by means of a lowpass filter including an amplifier 3, resistors R1 and R2, a diode D1 and a capacitor C1, as a rectifying detector. Also, in case that the frequency of the input signal from the input line 8 is high, it is impossible to achieve the system by making use of an universal operational amplifier because of the limitation in slew rate of the operational amplifier. Further, because the rectifying detection means 10 detects a maximum value of input signals during charging and discharging with a charging time constant R1C1 and a discharging time constant R2C1, it becomes impossible to carry out a stable operation unless an adequate time constant is arranged in accordance with the frequency of input signal. Furthermore, when impulse noises are included in the input signal, the aforementioned automatic gain control system fails to improve the noise problem because the maximum value of noise is detected and the maximum value is used to control gain of the amplifier.